1. Field of the Invention
The present invention is generally related to heat pumps and, more specifically, to a system and method for controlling the desuperheater function of a heat pump.
2. Description of Related Art
The origin of heat pumps in the United States can be traced back to as early as 1947. However, the commercialization of heat pumps did not gain any market penetration until the mid-1970's. Since that time they have continued to gain market share in the overall residential and commercial application for the primary source of heating and cooling for buildings.
The HVAC (heating, ventilating, air conditioning) industry has accepted the term “heat pump” to mean an appliance used in residential and commercial buildings for space heating and cooling. With reference to FIG. 1, a conventional heat pump 1 includes an evaporator 3 and a condenser 5, whereby heat is absorbed in evaporator 3 (heat exchanger) and released through condenser 5 (heat exchanger). Heat pump 1 can be changed using a reversing valve 7 so that evaporator 3 becomes a condenser and the condenser 5 becomes an evaporator (heat flow is reversed). This heating and cooling process is accomplished by using a vapor compression system utilizing various types of synthetic chemicals called refrigerants (i.e., fluids that can be changed easily from a liquid to a vapor, and from a vapor to a liquid, and are used as a heat transfer medium). Accordingly, heat pump 1 includes five major components: a compressor 9, refrigerant control device (not shown), reversing valve 7, condenser 5, and evaporator 3.
Heat pump 1 is charged with refrigerant in a closed refrigerant circuit, whereby, when compressor 9 (pump) is turned on, it draws (suction) refrigerant gas into a suction port 11 of compressor 9 where it is compressed and pumped out of a discharge port 13 of compressor 9 as a superheated gas at high pressure. This high pressure superheated gas travels to condenser 5 where it is cooled (i.e., gives up its heat) and, as a result, condenses into a liquid refrigerant. The liquid refrigerant is then forced by the pumping action of the compressor 9 to flow through a refrigerant control (i.e., a metering device) into the evaporator 3. The liquid refrigerant, having traveled through the metering device, is now subject to the suction (vacuum side) of compressor 9. As the liquid refrigerant enters the evaporator 3, the low pressure (i.e., suction of the compressor) causes it to evaporate into a cold vapor because such a change is accomplished by a change in heat content, absorbing any heat that is present in evaporator 3. This refrigerant gas, having absorbed heat from evaporator 3, enters suction port 11 of the compressor where the cycle is repeated.
Refrigerant vapor compression systems and specialty heat pumps have the ability to raise the refrigerant discharge gas temperature above 180° F. This superheated refrigerant gas can be used to transfer some of its heat content to water for use in domestic water heating. This can be accomplished by installing a heat exchanger designed for interchange of heat between superheated refrigerant gas and the water. This heat exchanger is commonly referred to as a desuperheater 15. By design, desuperheater 15 can capture only a portion of the heat of compression, while at the same time doing its intended refrigeration job. The desuperheater 15 is installed in the refrigerant discharge line between compressor 9 and condenser 5/evaporator 3.
In heat pump 1, about 25% of the work of condenser 5 is in reducing the superheated refrigerant gas temperature to the condensing temperature. The balance of the work of condenser 5 is in condensing the gas to a vapor and then further subcooling it to a liquid. By design, desuperheater 15 must be sized so that it will only remove some of the superheat (e.g., no more than 25% of the total load on condenser 5). Sizing of desuperheater 15 is critical with heat pumps, for an oversized desuperheater 15 will condense the superheated gas, robbing capacity from the air side condenser which would lower the heat output capacity and air delivery temperature in the space heating mode. With reference to FIG. 2, when a desuperheater 15 is provided in a heat pump refrigerant circuit, the addition of an insulated hot water storage tank 17 is essential for storing hot water to be used later as needed. In addition to the storage tank, a hot water circulator pump 19 is needed to pump the water from storage tank 17 to desuperheater 15 where it is heated and returned to storage tank 17.
Common industry practice in refrigeration equipment and, more specifically, heat pumps, is to control this desuperheating of hot water by the following control method. An electromechanical strap-on aquastat is provided that senses the water temperature leaving desuperheater 15. The aquastat turns on hot water circulator pump 19 when the water temperature drops to a cut-in temperature (e.g., 95° F.) and turns off hot water circulator pump 19 when the leaving water temperature reaches the cut-out temperature (e.g., 120° F.). Power to circulator pump 19 is supplied by a compressor contactor (relay) as illustrated in the wiring diagram of FIG. 3. Thus, circulator pump 19 could not run unless compressor 9 was energized. This control method works, but can be very inefficient and, in some cases, actually works in reverse of intended energy savings. With heat pumps, and more specifically water source heat pumps, there are times when the heat pump discharge gas temperature can be lower than the desired temperature in hot water storage tank 17. If this condition occurs, heat in storage tank 17 can actually be transferred from storage tank 17 to the discharge gas back into condenser 9 of heat pump 1.
Accordingly, a need exists for a method and system for controlling a desuperheater that overcomes some or all of the drawbacks and deficiencies evident in the prior art and described above.